Technical Field
The present invention relates to electronic, optical and optoelectronic devices and their fabrication, and more particularly to methods and devices for separating and isolating a graphene monolayer.
Description of the Related Art
Microelectronics applications have been increasingly employing graphene devices due to their conductive, mechanical and other properties. Processing of graphene is often unconventional relative to traditional processing techniques and is often difficult to control and incorporate into semiconductor processing. Graphene may be obtained using many techniques. One popular technique includes micro-mechanical exfoliation of graphite. This includes employing adhesive tape to repeatedly split graphite crystals into increasingly thinner pieces. The tape with attached optically transparent flakes is dissolved in acetone, and the flakes including monolayers are sedimented on an oxidized silicon wafer. This has been improved by dry deposition, avoiding the stage when graphene floated in a liquid. This is often referred to as a “scotch tape” or drawing method. This technique may not produce a uniform graphene film.
Another method of obtaining graphene is to heat silicon carbide (SiC) to high temperatures (>1,100° C.) under low or high vacuum to reduce it to graphene. This process produces epitaxial graphene with dimensions dependent upon the size of the SiC substrate (wafer). The face of the SiC used for graphene formation, silicon- or carbon-terminated, highly influences the thickness, crystal orientation, mobility and carrier density of the graphene. Exfoliation and transfer of the graphene layer is often very difficult.
Other methods use the catalytic action, carbon solubility, and atomic structure of a metal substrate to seed the growth of graphene (epitaxial growth). In one technique, copper foil is employed as the metal catalyst, and, at very low pressure under a carbon-containing gas, the growth of graphene automatically stops after a single graphene layer forms. Arbitrarily large graphene films can be created by chemical vapor deposition (CVD) growth. Multilayer graphene may also form on copper, in some areas of the substrate. However, exfoliation and transfer of the graphene layer is quite difficult. Another disadvantage of CVD growth on metal substrates is that the grown graphene layer is a poly-crystal, with randomly oriented domains.
In many of these techniques and especially with the graphitization of SiC, a top surface of the graphene may contain a double layer. This is due to the existence of vicinal terraces or other defects on the surface of the SiC substrate, areas where the growth rate is enhanced. The double layer graphene that forms along the edge of a terrace negatively impacts performance, by e.g., decreasing carrier mobility.